Handling and Surface Modificaiton pf Ultrafine Particles by CVD in Fluidized Bed Under Reduced Pressure

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Chemical vapor deposition method is very attractive to produce ultrafine particles with controlled chemical and crystalline compositions. One of the most important factors is how to handle fine particles in the gas phase. In this study, fluidization, which is normally used only for coarse particles, is refined for the handling of ultrafine particles.(1) Si_3N_4, SiC, Al_2O_3 and TiO_2 particles of 30-400 nm diameter were fluidized under 0.1-100 kPa and 25-200^ﾟC, and characteristics of bubbles, entrainment and mixing were evaluated by using optical fiber probes and pressure sensors equipped at various positions in the bed. The fluidization gas was dry air or nitrogen, and the gas velocity was larger than 1 cm/s. Cohesive particles were also fluidized by mixing coarse particles as the turbulent promoter. The fluidizability was correlated from the view of physicochemical properties of particles.(2) CVD processing were actually carried out in a bench-scale fluidized bed of 28 mm diameter. The coating of Si_3N_4 or Al_2O_3 with electro conductive TiN by the reaction of NH_3 and TiCl_4, and the coating of Al_2O_3 with thermo-conductive AIN by the reaction of NH_3 and AlCl_3 were among them. The results show that the fluidization is very effective to modify ultrafine particles by CVD reactions.